1. Field of the Invention
This invention relates generally to technologies for switching and routing optical wavelengths, and more particularly, this invention relates to waveguide grating-based wavelength selective switches and to add/drop devices comprising these wavelength selective switches.
2. Description of the Related Art
Optical wavelength division multiplexing (WDM) is a very important method used in modern optical fiber communication systems to dramatically increase the data transmission rate. In WDM systems, the whole optical beam consists of a number of different wavelength optical signals (wavelength channels). Each wavelength channel carries its own data information transmitted over the fiber. Therefore, with WDM technology a single optical fiber can transmit a number of distinguishable optical signals simultaneously. The result is a significant increase of effective bandwidth of the optical fiber and data transmission rate of the communication system.
In the WDM networks of the past, adding, dropping or cross connecting of individual wavelengths has involved conversion of the optical signal back to the electrical domain. Development of all-optical switches for applications ranging from add-drop functionality to large-scale cross-connects is key to adding intelligence to the optical layer of the optical networking systems. However, with the current technical limitations, all fiber network systems implemented with optical switches are still quite expensive.
To employ WDM technology in an optical communication system, optical demultiplexers, switches, multiplexers, and add/drop devices are important. Current state of the art in optical switching and signal transmission systems are limited to optical switching of an entire spectral range without wavelength differentiation or selection. Due to the lack of wavelength selection, an optical switch operation must frequently operate with a wavelength de-multiplexing and multiplexing device to achieve the transfer of optical signals of different wavelengths to different ports. This requirement leads to more complicated system configurations, higher manufacture and maintenance costs, and lower system reliability. For this reason, even though optical switches provide an advantage that the optical signals are switched entirely in the optical domain without converting them into the electrical domain, the cost and size of system cannot be easily reduced.
An add/drop device is used to inject (add) or extract (drop) one or more wavelength channels to or from a WDM network. Current optical add/drop devices usually consist of various types of optical switches and require optical multiplexers and demultiplexers, as shown in FIGS. 1A and 1B. FIG. 1A shows a typical block diagram of an optical add/drop device. Through the optical add/drop device, wavelength channels can be added or dropped to or from the main optical transmission trunk. FIG. 1B illustrates the construction of a typical prior-art optical add/drop device. This optical add/drop device requires a demultiplexer and a multiplexer to carry out wavelength selective switching operations in order to accomplish the add/drop functions. The requirement of a demultiplexer and a multiplexer makes the prior-art optical add/drop devices complex and costly to build. For a simple add/drop matrix, this requirement of a demultiplexer and a multiplexer is a significant burden. In addition, for a larger add/drop matrix, these prior-art optical add/drop devices suffer from their rapidly increasing complexity as the matrix size grows.
Due to the requirement of optical multiplexers and demultiplexers as well as functionality limitation of these optical switches, optical add/drop devices built upon these optical switches usually suffer from complexity, inflexibility, and high cost.